The present invention relates to an etching method and etching liquid which are suitable for forming minute wiring of silver or silver alloy for a semiconductor device, a liquid crystal display, an IC card and so on.
Electrode wiring in a semiconductor or a liquid crystal display has been processed minutely increasingly. As an electrode wiring material, silver and silver alloy having a high index of reflection and low electrical resistance have been used widely.
Silver and silver alloy have an index of reflection higher than that of aluminum and aluminum alloy and are thus widely used as the reflecting electrode wiring materials for semiconductors, liquid crystal displays, and solar cells. To form electrode wiring made of silver or silver alloy, silver or silver alloy is processed into a predetermined pattern. Examples of a method for forming the pattern are a wet etching method using chemicals and a dry etching method such as ion etching and plasma etching.
The wet etching can be conducted less expensively with using cheap chemicals and without an expensive apparatus than the dry etching. According to the wet etching method, a metal material having various kinds of outer profile can be processed, and a material having a three dimensional structure can be processed. The wet etching has still another advantage that silver, silver alloy and compounds thereof can be easily recovered from etching waste liquid, and the wet etching has been widely conducted.
A known etching liquid for silver or silver alloy includes an iron nitrate solution, a solution containing iodine and iodine compound, a solution containing hydrogen peroxide and ammonia, and a solution containing nitric acid. Among these etching liquids, the solution containing nitric acid is well reactable to silver or silver alloy so as to provide higher etching speed, and the solution is easy to be handled.
These etching liquids are also used for forming a pattern formed by using photolithographic technique for fine electrode wiring made of silver or silver alloy for semiconductors and liquid crystal displays.
An etching liquid described in Japanese patent publication H9-208267A contains hydrochloric acid and a surfactant in order to remove silver or silver alloy residue which is hardly dissolved and to prevent such residue from adhering to a surface of a substrate again.
A substrate for a liquid crystal display is manufactured by a process including a step of forming a silver or silver alloy thin layer having a predetermined thickness on a main surface of a glass substrate by a sputtering method; a step of forming a resist resin layer for forming a fine electrode pattern or wiring pattern by using photolithographic technique; a step of removing the silver or silver alloy thin layer which is not covered with the resist resin layer by wet etching method; and a step of removing the resist resin layer.
In the etching step of removing the silver or silver alloy thin layer having the fine pattern by the photolithographic technique, the silver or silver alloy should be etched uniformly without producing etching residue of the silver or silver alloy, while preventing etching excessively a side periphery of the silver or silver alloy layer covered with the resist resin layer. Etching the side periphery is called as xe2x80x9cside etchingxe2x80x9d.
However, according to conventional wet etching technique, silver or silver alloy is easily left at portions where a vertical line and a lateral line formed by etching cross, that is, narrow portions surrounded by silver or silver alloy layer and resist resin layers. This means that etching residues are easily produced there. To avoid this problem, the etching process is continuously conducted until no residue exists. This leads to xe2x80x9cover etchingxe2x80x9d where the silver or silver alloy is etched for a longer period of time to manufacture the wiring having the predetermined width. Namely, xe2x80x9cover etchingxe2x80x9d is conducted. Although the over etching prevents the residue, it proceeds the side etching of the silver or silver alloy thin layer, thus leading to uneven etching.
In the etching method of the present invention, a silver or silver alloy thin layer on a substrate surface is etched by steps of supplying an etching liquid into an etching tank or an etching liquid feeding apparatus, wherein the etching liquid contains silver ions in a range from 0.005 to 1 weight %, and contacting the layer with the etching liquid.
The etching liquid for silver or silver alloy of the present invention can contain at least phosphoric acid, nitric acid, acetic acid, and silver ion, wherein the silver ion concentration is in a range from 0.005 to 1 weight % and the acetic acid concentration is 1 weight % or more.
According to the present invention, in wet etching for a sliver or silver alloy thin layer, the residue of silver or silver alloy is prevented and the side etching due to over etching is also prevented. In addition, even and stable etching can be conducted.
It is well known in the art that when ions of a metal as the etching subject exist in etching liquid, acid components for dissolution (e.g. phosphoric acid, acetic acid) and oxidant components (e.g. nitric acid) in the etching liquid are consumed, thus lowering the etching rate. Accordingly, the conventional etching liquid contains no ion of a metal as the etching subject when the etching liquid is used at the start of etching (i.e. the etching liquid is fresh). In addition, the used etching liquid is normally discarded because the metal ion therein is hardly removed.
In contrast, the etching liquid of the invention contains silver ions in an amount from 0.005 to 1 weight % even when it is fresh. In addition, during etching, the silver ion concentration in the etching liquid is always kept preferably in a range from 0.005 to 1 weight %, thereby preventing residue of silver or silver alloy at etched portions and excessive side etching due to over etching. Therefore, uniform and stable etching can be conducted.
The etching method of the present invention has supreme effects in etching for forming reflecting electrode wiring of a semiconductor, a liquid crystal display, a solar cell, or the like. The method can be applied effectively to etch silver or silver alloy portions on the substrate for a fine wiring range which is uncovered with the resist while most of the substrate surface is covered with the resist resin layer. The silver or silver alloy can be etched uniformly without the side etching, without leaving residues in the fine wiring range, and without over etching.
The silver ion concentration in the etching liquid is always kept in a range from 0.005 to 1 weight % by holding the etching liquid having a silver ion concentration in a range from 0.005 to 1 weight % in a tank or an etching liquid feeding apparatus and bringing the liquid into contact with the silver or silver alloy layer on the substrate when the etching process is started. The liquid is also kept its silver ion concentration in the above range during the substrate is immersed in the liquid, so that the liquid having the ion concentration of the above range is always made contact with the silver or silver alloy thin layer.